No, those studies have been debunked. If you search up newer studies you will see that this is largely because of wrong assumptions, studies not being longitudinal and the default hypothesis comparing certain underdeveloped brain regions in children. They found what they wanted to find: That underdeveloped brains were similar to ADHD brains in certain regions. Does an ADHD child grow out of it just because they learn coping mechanisms and stop hanging in the curtains? Ofc not. They often develop anxiety, depression, OCD and addictions. There is a massive upsurge of adult diagnosing these days. Another point, there were frequent misdiagnosing, especially in the late nineties because ADHD was often just based on normal, hyper or inatentive behaviour in a lot of neurotypical, but often troubled children.

By what age? Genuinely curious.